So far well known methods of improving the heat resistance of rubber materials includes addition of silica as a white filler. The present applicant has so far proposed such methods. Besides, several other methods have been also proposed:                Patent Literature 1: JP No. 3704986        Patent Literature 2: JP-A-61-225243        Patent Literature 3: JP-A-2004-75831        Patent Literature 4: JP-A-2005-272580        
However, the afore-mentioned Patent Literature 1 proposes use of white carbon having a specific surface area (by nitrogen adsorption method) of not more than 200 m2/g, preferably about 30 to about 130 m2/g, where the rubber materials, to which the white carbon is to be added, is not NBR, but hydrogenated NBR. The afore-mentioned other Patent Literatures make no specific mention of specific surface area of silica.
Furthermore, a rubber composition, which comprises natural rubber or synthetic rubber, a silica-based, reinforcing filler having a specific surface area of not less than 30 m2/g, an acetylenic alcohol (alkylene oxide adduct), and a specific organopolysiloxane, has been also proposed, where the synthetic rubber is exemplified by NBR or hydrogenated NBR, and in Examples precipitated silica (Nipsil LP) having a specific surface area of 230 m2/g is used, but the significance of using smaller specific surface area is nowhere emphasized.                Patent Literature 5: JP No. 2674630        
Furthermore, it has been also proposed to add silica (BET method specific surface area: 70-160 m2/g) to NBR for use in vibration-damping rubber or to add silica (BET method specific surface area: 50-400 m2/g) to NBR-containing blend material for use in belts or tires, though their uses are different from the use in seal materials. On the other hand, the compression set characteristics are an important factor for the use in seal materials.                Patent Literature 6: JP-A-2004-217849        Patent Literature 7: JP-A-2000-281839        